I don't know about a 40% decrease, though it is a pretty large decrease in power consumption. This is because the 40GB is built with a 65nm cell processor, while other models are built with a 95nm cell processor. Though the PS3 only takes up about as much power as an average PC, so it doesn't really take a hit on your power bill in the first place.

Though you also want to take into consideration that the 40GB model doesn't play PS2 games, while the 80GB model does. Also, sony will start making all models with 65 or possibly 45nm chip sets soon.